1. Field of the Invention
The present invention relates to a wireless communication apparatus and a wireless communication method using a variable transmission rate, and for example, to a wireless communication apparatus that aggregates a plurality of frames for transmission.
2. Description of the Related Art
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 Committee is now formulating IEEE 802.11n standards that are next-generation high-speed wireless local area network (LAN) standards.
The IEEE 802.11n standards adopt Aggregate-MPDU (A-MPDU) that allows a plurality of MAC protocol data units (MPDUs) to be transmitted as a single PHY protocol data unit (PPDU). Bringing a plurality of MPDUs together into a single PPDU is called aggregation. In the specification, if MPDU and A-MPDU are not distinguished from each other, these units are simply called frames. The use of A-MPDU is expected to enable transfer that is more efficient than short inter-frame space (SIFS) burst transfer, which is most efficient in all the conventional standards including the latest IEEE 802.11e standards. Such a technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-352844.
However, frame transmission efficiency generally decreases with increasing frame length. If the transmission efficiency decreases to prevent accurate transmission, the same frame needs to be transmitted again (hereinafter referred to as “retransmission”). The time required for retransmission increases consistently with the frame length.
As a result, the transmission efficiency may not be sufficiently improved simply by using A-MPDU.